1. Field of the Invention
The present invention relates to electrographic apparatus, methods and systems that employ a small particle developer mixture of hard magnetic carrier and electrically insulative toner and more specifically to techniques and structures for adjusting the development (e.g. the density and/or contrast) of electrographic images.
2. Description of the Prior Art
U.S. application Ser. No. 510,109, entitled "Improved Electrographic Development Method, Apparatus and System, and filed July 1, 1983, in the name of Fritz et al, discloses a system wherein improved electrographic development is obtained, in the presence of a development electrode field, by predeterminedly rotating the core and shell of a magnetic brush applicator that supplies a small particle developer mixture (comprising hard-magnetic carrier and electrically insulative toner) to a moving electrostatic image member. In one preferred embodiment the core and shell are cooperatively rotated so that toner plated-out on the shell does not adversely affect image development and so that the developer has a generally equal and co-current linear velocity to the image member.
In the development method and apparatus disclosed in that application, the amounts of toner transferred to the developed electrostatic image portions can be adjusted in the conventional manner, viz. by controlling the electrical bias of the development electrode. The density and contrast characteristics of the developed electrostatic image also can be varied by adjusting other parameters of the electrophotographic system. For example, U.S. Pat. No. 4,350,435 discloses apparatus for cooperatively controlling the primary charge level, the image exposure level and/or the magnetic brush bias level to control the contrast and density of copies, i.e. for adjusting the input original density (D.sub.in) to output copy density (D.sub.out) characteristic (gamma curve) of the electrophotographic system.
Although the traditional development electrode and the contrast/density control of the types disclosed in U.S. Pat. No. 4,350,435 are highly useful in apparatus employing development systems such as disclosed in the Fritz et al application, it is desirable to have additional flexibility in adjusting the D.sub.in /D.sub.out characteristics of electrophotographic systems. I have found that the development system disclosed in the Fritz et al application is susceptible to a unique development control approach which provides such additional image development adjustment flexibility.